1. Field of the Invention
The present invention relates to a light pipe and a surface light source unit that can be used for forming a reflection type liquid-crystal display device which is excellent in light utilizing efficiency and which is bright and easy to view.
The present application is based on Japanese Patent Applications No. Hei. 11-319909, which is incorporated herein by reference.
2. Description of the Related Art
A surface light source unit permitting a reflection type liquid-crystal display device to be viewed in a dark place, or the like, has been in great demand. In the meantime, the inventor of the present invention has tried to apply a front-lighting system in which a back-lighting type surface light source unit used in a transmission type liquid-crystal display device is disposed on the viewing side of a liquid-crystal cell. Such a back-lighting type surface light source unit uses a light pipe provided with a light output means so that light incident on a side surface exits from either one of the upper and lower surfaces through the light output means.
In the front-lighting system, however, the contents of display are viewed through the light pipe. Hence, in the background-art surface light source unit using such a back-lighting type light pipe, there was a problem that display was hard to view because of shortage of contrast in use of external light both at the time of switching-on and at the time of switching-off. The shortage of contrast was mainly caused by over lapping between display light and reflected light of leaking light and external light at a surface of the light pipe and low light output efficiency of the light pipe.
An object of the present invention is to develop a light pipe and a surface light source unit that can be adapted for a front-lighting system and used for forming a reflection type liquid-crystal display device which is excellent in contrast in viewing both at the time of switching-off and at the time of switching-on, excellent in brightness of display and excellent in clarity.
According to the present invention, there are provided: a light pipe comprising a plate-like member having a refractive index in a range of from 1.51 to 1.55, the plate-like member having an upper surface, a lower surface, and an incidence side surface, the plate-like member further having a light output means constituted by a repetitive prismatic structure and formed in the upper surface so that light incident on the incidence side surface exits from the lower surface through the light output means but light incident on the lower surface is transmitted through the upper surface, whereby information light generated on the lower surface of the light pipe is transmitted to the upper surface of the light pipe so as to be made visible at the upper surface; a surface light source unit in which a light source is disposed on an incidence side surface of the above-mentioned light pipe; and a reflection type liquid-crystal display device in which a liquid-crystal cell including a reflection layer is disposed on a lower surface of the above-mentioned light pipe or on a lower surface of a light pipe contained in the above-mentioned surface light source unit.
According to the present invention, a light pipe and a surface light source unit used for a front-lighting system can be obtained to thereby make it possible to form a reflection type liquid-crystal display device which is excellent in contrast in viewing both at the time of switching-on and at the time of switching-off, and which is excellent in display brightness and clarity. This is because the refractive index of the light pipe is set to be in a range of from 1.51 to 1.55 so that leaking light or reflected light of external light at a surface of the light pipe is balanced with light output efficiency of the light pipe.
That is, when the light pipe is made of a low refractive index material such as polymethyl methacrylate having a refractive index of 1.485, the light pipe lacks light output efficiency in display at the time of switching-on. Hence, because a large amount of light leaks from the surface of the light pipe, display becomes so dark that contrast is apt to run short. When the light pipe is made of a high refractive index material such as polycarbonate having a refractive index of 1.59, surface reflection overlapping with display light in display using external light is large. Hence, display becomes so dark that contrast is apt to run short. Therefore, when a light pipe having a refractive index in a range of from 1.51 to 1.55 is used, surface reflection overlapping with display light can be suppressed so that shortage of contrast in display using external light can be prevented. Moreover, the angle range permitting transmission light to be fully reflected is so wide that the light output efficiency can be improved. Moreover, light leaking from the surface of the light pipe can be suppressed so that shortage of contrast in display also at the time of switching-on can be prevented.
Further, when the light output means of the light pipe is constituted by a repetitive prismatic structure each consisting of a combination of a short side surface and a long side surface having predetermined inclination angle ranges respectively, the light pipe can be obtained as a light pipe providing output light excellent in perpendicular directivity. Hence, there can be obtained a surface light source unit which is excellent in the quantity of light in a frontal direction perpendicular to a liquid-crystal cell so that brighter display at the time of switching-on can be achieved and which is excellent in light utilizing efficiency for a front-lighting system because display image through the light pipe is hardly disordered. Hence, there can be obtained a reflection type liquid-crystal display device excellent in display quality.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.